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D( x )
3*x+3 = 0
3*x+3 = 0
3*x+3 = 0
3*x+3 = 0 // - 3
3*x = -3 // : 3
x = -3/3
x = -1
x in (-oo:-1) U (-1:+oo)
y/(3*x+3) = 30 // - 30
y/(3*x+3)-30 = 0
y/(3*x+3)+(-30*(3*x+3))/(3*x+3) = 0
y-30*(3*x+3) = 0
y-90*x-90 = 0
(y-90*x-90)/(3*x+3) = 0
(y-90*x-90)/(3*x+3) = 0 // * 3*x+3
y-90*x-90 = 0
y-90*x-90 = 0 // - y-90
-90*x = -(y-90) // : -90
x = (-(y-90))/(-90)
x = (90-y)/(-90)
x = (90-y)/(-90)
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